RNAi is a powerful cellular mechanism that uses short RNAs to target homologous transcripts for destruction. This is an ancient mechanism found in organisms ranging from fungi and plants to mammals. It most likely is a primitive innate immune response mechanism that can protect organisms from viral infection and retro-transpose on movement. The process is initiated in vivo by 21 to 23 nucleotide duplexes with 2 base 3' overhangs that are generated from longer RNAs by an enzyme called Dicer. These short duplexes are termed siRNAs, and they become associated with a protein complex called the RNA induced silencing complex, or RISC, that directs a single strand of the duplex to a complementary target sequence, resulting in degradation of the target. Several investigative groups have recently reported impressive inhibition of HIV infection using synthetic siRNAs or intracellular expressed siRNAs. RNAi appears to be a powerful, target specific inhibitory mechanism that has potential application for the therapeutic treatment of HIV-1 infection. The present study capitalizes upon very exciting and promising preliminary findings that demonstrate vector driven siRNAs can effectively block HIV infection in primary lymphocytes. The overall goal of this project is to test the hypothesis that siRNA can be used as an anti-HIV-1 agent in hematopoietic stem cell gene therapy. The research focus of this proposal is the optimization of siRNA function and expression from lentiviral vector backbones. New expression systems that deliver siRNAs to the cellular cytoplasm will be tested, along with the possibility that siRNAs can be used to direct gene silencing through DNA methylation. These inhibitory molecules will be targeted to HIV-1, common sequences in SHIV, and the cellular co-receptor CCR5. Mechanisms of action studies are proposed that will allow the development of the most efficacious strategies for deployment of siRNAs in human gene therapy. Finally, the combined use of siRNAs and other RNA based therapeutics will be tested for enhanced efficacy and minimization of viral resistance. The issue of potential toxicity will be tested in long-term bone marrow cell culture and via testing in fetal Rhesus monkeys in collaboration with Dr. Alice Tarantal in an accompanying proposal. The specific aims of this project are as follows, [unreadable] 1) Optimizing expression of anti-HIV siRNAs from lentiviral and retroviral vector backbones.2) Testing the mechanism of action of anti-HIV siRNAs.3) Evaluation of combinations of siRNAs and siRNAs combined with anti-HIV ribozymes and decoys. The long range goal of these studies is to develop an siRNA based therapy for the treatment of HIV-1 infection in a gene therapy setting capitalizing on the infrastructure this program has developed for ribozyme based gene therapy. [unreadable] [unreadable] [unreadable]